Add methods to select and deselect SPI slaves

[hannobraun]

Based on #256. I recommend reviewing/merging that one first.

[hannobraun]

Hold on, this is not quite right. It turns out that you can't deselect a slave using the method provided here. You do so by setting EOT (end of transfer) in TXCTL or TXDATCTL.

This might mean that we can't provide this feature and implement spi::FullDuplex at the same time, as there's no way for FullDuplex::send to know that it's writing the last byte. And setting EOT in a separate method will not take effect until we write another word, and would be of no use to a driver anyway.

Probably best to close this PR and handle slave select via GPIO (which I think drivers in the wild already do).

[hannobraun]

I'm closing this pull request, as I suggested in my previous comment. It's partially wrong and largely unnecessary in master mode. I've incorporated the parts of it required for slave mode into my WIP branch.